Table of Contents
1.0
Introduction
1
1.1
What is an Organic Peroxide
2
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Polymers that can and cannot be crosslinked
Peroxide vs. Sulfur Donor Systems
Peroxide Classification, Dialkyl Peroxides
Dialkyl Peroxides vs. Peroxyketals
Diacyl Peroxides
Active Oxygen and Percent Assay
Half-life
Decomposition By-Products Summary
3
4
5
6
8
8
9
10
2.1
2.2
Processing
Processing Time (Scorch)
Cure Time and Crosslinking Efficiency
12
13
2.0
3.0
Chart: Specifications; Half Life Temperatures;
Compounding Information
4.0
Effect of Compounding Ingredients
Antidegradants
Plasticizers
16
16
Coagents
Silicone Rubber
FDA Checklist
Safety Checklist Summary
References and Trademarks
17
22
25
27
29
4.1
4.2
5.0
6.0
7.0
8.0
9.0
14
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and Material Safety Data Sheets.
vanderbiltchemicals.com
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E-Mail: rubber@vanderbiltchemicals.com
Before using, read, understand and comply with the information and precautions in the Safety Data Sheets, label and other product
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regarding accuracy, performance, stability, reliability or use. This information is not intended to be all-inclusive, because the manner and
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rev09/17/2014
Comprehensive VAROX® Peroxide Accelerator
Product Guide
Introduction
Vanderbilt Chemicals, LLC began distributing VAROX organic peroxides in the late
1950’s. They have since been marketed throughout the world and promoted in several
editions of The Vanderbilt Rubber Handbook including the newest edition. Today, the
VAROX product line consists of over twenty grades that meet the various applications of
the Rubber and Plastics industries.
Beyond the product line itself, Vanderbilt Chemicals provides dedicated technical
support to its customers. The individuals of Vanderbilt’s technical sales force, which
covers the North American Rubber and Plastics markets, combine over 500 years of
technical expertise, and over 200 years of service with the company. Backing them up at
Vanderbilt’s headquarters in Norwalk, Connecticut are Rubber and Plastics Application
Laboratories, an Analytical Laboratory and a Research and Development Group. This
team stands ready to answer our customers’ technical questions with regard to Rubber
and Plastics polymers, additives and compounding.
In addition to the organic peroxide product line, Vanderbilt Chemicals supplies over 500
products to the Rubber and Plastics industries. For further information, please contact us
at 800.243.6064 / 203.853.1400, or consult our website at vanderbiltchemicals.com.
We hope that this peroxide brochure will be useful to both the novice and the
experienced compounder.
1
PEROXIDE CROSSLINKING of ELASTOMERS1
Crosslinking and/or vulcanization are defined as a process for converting a
thermoplastic material or elastomer into a thermoset or vulcanizate.1 This process
converts unbound polymer molecular chains into a single network which retains many
desirable physical and chemical properties of the base polymer. The two major
chemical processes by which crosslinking occurs are peroxide and sulfur cure systems.
Peroxide systems are more versatile since they can be used to crosslink both saturated
and unsaturated polymers, thereby providing a wider selection of elastomers, and more
opportunities for cost savings.
Peroxide crosslinking systems can:
• Offer a truly nitrosamine-free finished product with predictable cure rates and
cured physical properties.
• Provide a stable factory stock elastomer compound, as opposed to a
sulfur-cured compound with a short shelf life (sometimes days).
• Be made equivalent, and in many cases superior, to sulfur systems, by varying
the ratio of many common additives.
• Produce thermosets and vulcanizates having better heat aging properties,
lower compression set, less color, no reversion, reduced (if any) bloom, and
lower odor levels than compounds cured by sulfur systems.
What is an Organic Peroxide?
An organic peroxide is a molecule containing at least two oxygen atoms, connected
by a single bond to organic chemical groups, as shown below.
(VAROX® DCP Peroxide Accelerator Dicumyl Peroxide):
CH3
CH3
C O O C
CH3
CH3
Depending on the groups attached, this oxygen-oxygen bond is designed to break on
heating, leaving one unpaired electron on each oxygen, called a “free radical”. These
free radicals are able to promote certain chemical reactions, such as:
• Polymerization of one or more monomers
• Curing of thermosetting resins (polymer + monomer)
• Crosslinking of elastomers and polyethylene
Organic peroxides that are thermally decomposed generate free radicals that
consequently create an active site on a polymer backbone. The reaction between two
active sites creates a strong link between the polymer chains, forming a polymer network
exhibiting very desirable mechanical properties, particularly excellent heat resistance
and compression set. Another advantage of using a peroxide cure instead of sulfur
vulcanization is the wide range of polymers that can be crosslinked (unsaturated
polymers as well as saturated polymers like polyethylene). Due to the nature of the strong
carbon-carbon crosslink bond created by the use of organic peroxides, it is possible to
use the full engineering capabilities of these peroxide crosslinkable polymers. Tables 1
and 2 list the polymers that can and cannot be crosslinked by organic peroxides.
2
AEM
AU/EU
BIIR
BR
CM
CR
CSM
EBA
EEA
EPM
EPDM
EVA
FKM
HNBR
IR
NBR
NR
PE
POE
SBR
T
VMQ (MQ)
FVMQ
Table 1: Polymers Crosslinkable with Organic Peroxides
Poly(ethylene-co-methylacrylate) (Vamac®)
Polyurethane Rubber
Bromobutyl Rubber
Polybutadiene Rubber
Chlorinated Polyethylene
Polychloroprene Rubber (Neoprene)
Chlorosulfonyl Polyethylene
Ethylene Butylacrylate Copolymer
Ethylene Ethyl Acrylate
Ethylene Propylene Copolymer (Vistalon™)
Ethylene Propylene Diene Terpolymer (Vistalon™ )
Ethylene Vinylacetate Copolymer
Fluoroelastomers (Viton®)
Hydrogenated Acrylonitrile-butadiene Rubber
Polyisoprene Rubber
Acrylonitrile-butadiene Rubber (Nitrile Rubber)
Natural Rubber
Polyethylene (includes high, low and linear low density)
Polyolefin Elastomer (Exact®)
Styrene-butadiene Rubber
Polysulfide Rubber
Silicone Rubber
Fluorosilicone Rubber
ACM
CIIR
CO
Table 2: Polymers Not Crosslinkable with Organic Peroxides
Polyacrylate Rubber
Chlorobutyl Rubber
Epichlorohydrin Rubber
ECO
Epichlorohydrin Copolymer
IIR
PB
PBE
PIB
PP
PVC
Butyl Rubber
Polybutene
Propylene-based Elastomers (Vistamaxx™)
Polyisobutylene
Polypropylene
Polyvinylchloride
3
Peroxides* vs. Sulfur and Sulfur Donor Cure Systems
(*Organic peroxides will be referred to as peroxides in the rest of this bulletin.)
Advantages of crosslinking with peroxides instead of sulfur include:
•
•
•
•
•
•
•
•
•
•
Formation of radicals which generate carbon to carbon linkages.
Best retention of properties after heat aging.
True non-nitrosamine crosslinking.
Improved resistance to chemicals and oil.
Lower compression set, and improved resiliency.
Superior electrical properties, since zinc oxide is not required.
Crosslinks both saturated and unsaturated polymers.
Wide range of operating temperatures.
Superior color retention, i.e., no discoloration.
Wide variety of peroxide half-lives for crosslinking and processing.
Disadvantages of crosslinking with peroxides instead of sulfur include:
• Restrictions on some ingredients (no aromatic oils or highly acidic fillers).
• Lower hot tear. However, certain blends of peroxides and coagents provide
hot tear equivalent to sulfur cure.
• Bloom (some types of peroxides), although not as extensive as with most
sulfur cure systems.
• Oxygen inhibition (surface tack in forced hot air oven cure).
• Cost of curatives. Although not necessarily in regard to the newer low
nitrosamine accelerators and that of the entire formulation, particularly when
lower cost elastomers can be readily substituted. More additives are required
for a sulfur cure vs. a peroxide cure.
When polymers are crosslinked by peroxides, carbon to carbon bonds are formed
between individual polymer chains. The C−C bond is stronger and more thermally stable
than the S−S bond formed by elemental sulfur vulcanization. Efficient Vulcanizing (EV)
systems (low sulfur and sulfur donor cure systems) primarily form C−S type bonds. The
energy (kJ) or thermal stability of C−S bonds falls between that of S−S and C−C bonds.
Because of the overwhelmingly higher strength of the covalent C−C bond network, the
peroxide cure is the preferred crosslinking method to obtain optimum thermal stability
and superior compression set properties. Table 3 compares the typical results of three
standard cure systems for a 65 Shore A, black-filled, EPDM compound:
Test
Table 3: A Comparison of Three Standard Cure Systems
Elemental Sulfur
Sulfur Donor
Peroxide
Crosslink Bond Energy, kJ
Compression Set After 70
hrs. @ 212 °F, %
Elongation After 120 hrs. @
300 °F, % Retention
155 - 270 kJ
285 kJ
350 kJ
52%
28%
11%
42%
63%
75%
4
Peroxide Classification
All peroxides have a peroxy group (−O−O−). What makes certain peroxides more
reactive than others? The answer is simply the chemical composition of the rest of the
molecule. The general formula for organic peroxide is R1−O−O−R2, where R1 and R2
represent other chemical groups that are bonded to the −O−O− group. Depending on
the chemical structure of R1 and R2, the organic peroxides typically used for crosslinking
elastomers can be classified as either Dialkyl, Diacyl, Peroxyketal or Peroxyester. A brief
summary of these classes is provided in Table 4.
Class
Dialkyl
Dialkyl
Dialkyl
Diacyl
Table 4: Peroxide Classifications
Commercial
Advantages
Disadvantages
Product
Higher cost. Less efficient cure.
VAROX® DBPH No odor. No bloom.
Peroxide Accelerator
VAROX DCP
VAROX VC-Flake
VAROX DCBP-50
Paste
Peroxyketal
VAROX 231
Very efficient. Low cost. Odor (Acetophenone).
Very efficient. No odor. Bloom.
Fast cure in Silicone.
Scorchy. Low cure efficiency in
carbon black-filled systems.
Faster curing. Lower
Scorchy. Higher cost. Low
efficiency in saturated
temperatures. No
polymers.
bloom.
VAROX DBPH aliphatic dialkyl peroxide has a lower crosslinking efficiency compared to
the other dialkyls. This is due to the generation of a combination of high energy and lower
energy radicals. Lower energy radicals do not readily participate in crosslinking by
hydrogen abstraction. However, VAROX DBPH has several outstanding advantages. It
doesn’t create bloom, generates very little odor, and due to the safer decomposition
by-products, it is used extensively in FDA-approved indirect food contact and medical
applications.
Table 5 compares the crosslinking efficiency of three dialkyl peroxides on a molar basis
in a carbon black-filled general purpose EPDM compound:
Table 5: Dialkyl Peroxide Crosslinking Efficiency in EPDM
Peroxide Type
Parts per Hundred Rubber (phr)
®
7.0
--VAROX DCP-40KE
Peroxide Accelerator
VAROX 802-40KE
--VAROX DBPH-50
--Moving Die Rheometer, 1° arc @ 180 °C
MH, (dN•m)
14.6
18.6
tS0.4 (sec)
t′90 (min)
2.93
Moles of Peroxide
0.010
Mooney Viscosity @ 135 °C
Minutes to 5 pt. rise
7.8
5
---
4.3
---
--5.5
15.66
22.2
5.36
0.011
16.05
22.8
6.32
0.017
15.2
16.2
Peroxyketal vs. Dialkyl Peroxide Crosslinking Performance
Peroxyketal peroxides are widely used for crosslinking and polymer modification.2 They
have fewer half-lives than dialkyl peroxides, and therefore provide faster reaction times at
a given temperature. The peroxyketal peroxides should be considered as replacements
for dialkyl peroxides when lower curing temperatures are required in order to reduce
curing times. The type of elastomer or polymer to be crosslinked or modified will
determine if peroxyketals will be a suitable choice.
Peroxyketal peroxides are non-aromatic and in their pure form are liquid at room
temperature. Peroxyketals do not produce solid decomposition by-products and are
therefore non-blooming.
Peroxyketals generate a combination of weak and strong free radicals, based on
hydrogen bond dissociation energies, which means that peroxyketals are less efficient
than dialkyl peroxides when crosslinking saturated polymers or polymers with low levels of
unsaturation. The latter would require considerably higher concentrations of peroxyketal
peroxide to replace dialkyl peroxide. With minimal unsaturation present in the polymer,
hydrogen abstraction becomes important, and requires a high level of strong free
radicals, like those provided by dialkyl peroxides. However, in certain polymerization
systems, e.g., acrylic and styrenic, or in highly unsaturated elastomers, the peroxyketals
are equivalent in crosslinking efficiency to the dialkyl peroxides.
Figure 1 compares the performance of dialkyl and peroxyketal peroxides in the curing
of EVA, a saturated polymer which requires strong radicals to enable hydrogen
abstraction for crosslinking. In the examples below, VAROX® 231-XL PDR Peroxide
Accelerator and VAROX 230-XL PDR peroxyketals provide low crosslinking efficiency, but
fast cure rates. However, as shown in Figure 2, the proper selection of a crosslinking
coagent, in this case VANLINK™ TAC Coagent (triallyl cyanurate), improves the
crosslinking efficiency of peroxyketals while retaining the desired faster cure rate.
Peroxide Cure Efficiency
MH (in•lbs)
Peroxides evaluated on an equal m olar basis (0.01 m ole peroxide group)
50
45
40
35
30
25
20
15
10
5
0
VAROX® 802-40KE
Peroxide Accelerator
(1.69phr)
VAROX DCP-40KE
1 (2.70phr)
1VAROX DBPH-50
1 (1.45phr)
1VAROX 231-XL
1 (1.46phr)
150
160
170
180
185
190
200
Temperature (°C)
Figure 1: Crosslinking EVA (9% Vinyl Acetate) Without a Coagent
6
Productivity at Different Temperatures
t c 90 Cure Time (min)
Peroxides evaluated on an equal molar basis (0.01 mole peroxide group)
30
VAROX®802-40KE
Peroxide Accelerator
(1.69 phr)
25
20
VAROX 802-40KE
4
15
10
(2.70 phr)
VAROX 802-40KE
(1.45 phr)0
5
VAROX 802-40KE
0
(1.46 phr)L
150
160
170
180
185
190
200
46 phr VAROX 231-
Temperature (°C)
Figure 2: Crosslinking EVA (9% Vinyl Acetate) With a Coagent
Table 6 shows that VANLINK™ TAC Coagent increases the degree of crosslinking by
improving the efficiency of VAROX® 230-XL Peroxide Accelerator, while maintaining its
dramatically faster cure rate.
Table 6: Crosslinking EVA using VAROX® 230-XL and VANLINK™ TAC
Ingredients
Parts per Hundred Rubber
®
2.5
----VAROX 802-40KE Peroxide Accelerator
VAROX 230-XL
--4.5
3.0
----1.0
VANLINK™ TAC Coagent
ODR @ 165.5 °C (330 °F), 3 ° arc @ 180 °C
70.1
57.6
89.2
MH, (dN•m)
11.3
12.4
12.4
ML, (dN•m)
2.4
1.5
1.6
tS2 (min)
t′90 (min)
17
5.4
5.3
Summary of Peroxyketals
1.
2.
3.
4.
5.
6.
7.
Peroxyketal peroxide efficiency increases when used with coagents:
• Coagents add unsaturation to the system.
• Coagents equalize peroxyketals and dialkyl peroxide cures.
• Coagents can improve Mooney viscosity, scorch time, compression set,
hardness, and modulus.
• Peroxyketals exhibit equivalent efficiency to dialkyl peroxides in highly
unsaturated elastomers.
Peroxyketals have lower half-lives, and provide much faster cures (lower t′90) than
the dialkyl peroxides.
Peroxyketals provide the ability to cure at much lower temperatures, e.g., over
curing EPDM on plastic parts while avoiding warpage.
Peroxyketals are stable liquids at room temperature.
Peroxyketals do not bloom, since they do not contain any solid decomposition
by-products.
Peroxyketals are aliphatic, or non-aromatic in chemical composition.
Peroxyketals have low odor.
7
Diacyl Peroxides
Diacyl peroxides decompose to useful free-radicals and have the least amount of
decomposition by-products. In addition to the crosslinking of elastomers, diacyl peroxides
are used in a variety of applications that include the curing of unsaturated polyester
resins, and the manufacturing of PVC, polystyrene and polyacrylates.
The low half-life temperature of diacyl peroxides, i.e. 1 minute t ½ of 267°F, makes them
unacceptable from a processing point of view for most crosslinking applications.3
However, applications requiring low processing temperatures can take advantage of the
diacyl peroxide’s efficiency.
Diacyl peroxides are primarily used for the crosslinking of silicone rubber; in carbon
black-filled EPDM formulations they generally provide poor crosslinking performance,
probably because of the peroxide’s chemical sensitivity to the complex surface chemistry
of the carbon black. This peroxide generates very high energy radicals and is therefore
capable of the hydrogen abstraction of labile hydrogens on the silicone rubber. This
peroxide is therefore well-known as a “non-vinyl specific” curative in the silicone rubber
industry. One of the most common diacyl peroxides for crosslinking silicone rubber is Bis
(2, 4-Dichlorobenzoyl) peroxide (VAROX® DCBP-50 Paste Peroxide Accelerator), whose
chemical structure is shown below.
O
O
C O O C
Cl
Cl
Cl
Cl
This is an Acyl group, hence
the name Diacyl Peroxides
Active Oxygen Content and Percent Assay
Active Oxygen Content ─ each organic peroxide contains a certain amount of active
oxygen, usually between 2% and 12%. This is a good indication of the expected activity of
peroxides of the same class. The active oxygen content, A[O], is defined as the
percentage between the atomic mass of oxygen in each O−O bond and the molecular
weight of the peroxide.
Example: VAROX DCP-40KE has one O−O group and a molecular weight of 270.37; its
peroxide content is 40%, so its active oxygen content will be:
(1 x 16) x 0.40 x 100 = 2.37%
270.37
As a general rule, lowering the percent active oxygen of an organic peroxide, or reducing its assay, will increase its safety and ease of handling in the workplace. For example,
when pure liquid VAROX DBPH peroxide is extended on an inert calcium carbonate /
silica filler, a safer and lower active oxygen content product is produced. This free flowing
powder greatly improves safety and handling of the peroxide, while increasing the
accuracy and uniformity of the final peroxide concentration in the elastomer compound.
8
Percent Assay ─ percent active oxygen content should not be confused with percent
assay. Percent assay is the measure of active peroxide content. For example, VAROX®
DCP-40C Peroxide Accelerator contains 40% active dicumyl peroxide, with the remaining
60% consisting of calcium carbonate.
Half-life Time and Half-life Temperature
Decomposition rates of organic peroxides are reported in terms of half-life time or
half-life temperature. The half-life time of a peroxide, at any specified temperature, is the
time in which 50% of the peroxide has decomposed. Correspondingly, the half-life
temperature at any specified time is the temperature at which 50% of the peroxide has
decomposed in the specified time. Table 7 shows how the number of half-lives correlates
to the percentage of peroxide decomposed.
Table 7: Half-lives vs. Percent Peroxide Decomposition
Number of Half-lives
Percent Peroxide Decomposed
0
0
1
50
2
75
3
87.5
4
93.75
5
96.9
6
98.4
7
99.2
8
99.6
9
99.8
10
99.9
The rate of crosslinking produced by a free radical initiator is determined by its rate of
thermal decomposition. Half-life data are essential in the selection of the optimum
initiator for specific time/temperature applications.
Peroxide manufacturers commonly include the 1 hour and 10 hour half-life
temperatures in their product bulletins. However, it is often useful to express peroxide
stability in terms of 1 minute, 1 hour, and 10 hour half-life temperatures, i.e. the
temperatures at which 50% of the initiator has decomposed in 1 minute, 1 hour and 10
hours, respectively.
Since crosslinking is directly related to the amount of peroxide decomposed, at least 6
to 10 half-lives of peroxide decomposition are recommended for crosslinking operations.
One mole of crosslinked peroxide equates to one mole of decomposed peroxide. The
t′90 (min) value is the time necessary to achieve 90% of the final cure. Thus, the t′90 (min)
is equivalent to 90% peroxide decomposed, or approximately 3.33 half-lives. The percent
of peroxide decomposed can be calculated by using the number of peroxide half-lives in
the equation on the next page:
9
Percent of Peroxide Decomposed = (1 – 0.5N) x 100
(Where ‘N’ is the number of peroxide half-lives)
•
•
•
At N = 3.33 half-lives, approximately 90% of the peroxide is decomposed.
At N = 6 half-lives, the peroxide is 98.4% decomposed.
At N =10 half-lives, the peroxide is almost completely decomposed at 99.9%.
Example 1: Calculate an estimated t′90 cure time for VAROX® DCP Peroxide Accelerator
at 340°F.
At a temperature of 340°F (171.1°C), dicumyl peroxide has a half-life time of 1.87
minutes. Applying the principle that t′90 (min) should relate to 3.33 half-lives of peroxide
decomposition, the t′90 cure time can be estimated.
Estimated t′90 (min) = 3.33 half-lives x 1.87 min. = 6.23 minutes
Example 2: Calculate the minimum cure cycle for VAROX DCP at 340°F.
Six half-lives is the minimum number of peroxide decompositions for a crosslinking cure
cycle. The theoretical minimum cure cycle is therefore 11.2 minutes (6 half-lives x 1.87).
This time assumes an isothermal profile of 340°F, with zero warm-up time.
For best performance it is recommended that as much of the peroxide as possible be
decomposed. To decompose 99.9% of the peroxide requires going through 10 half-lives.
This is especially true in the manufacture of gaskets and seals, where compression set is
important. Residual peroxide remaining in the rubber could react under further heat and
stress, resulting in an undesirable increase in percent set values. The theoretical time to
decompose 99.9% of the dicumyl peroxide would be 18.7 minutes (10 half-lives x 1.87
minutes) at 340°F.
Decomposition By-products
As shown in Table 8, organic peroxides decompose to form various types of
by-products. The type and quantity of decomposition by-products depend on the
amount of the specific peroxide used, together with the processing conditions.
Postcuring of the crosslinked product will reduce the residual amount of decomposition
by-products.
Table 8: Peroxide Decomposition By-Products
Peroxide
Major Decomposition
Minor Decomposition
Products (Hypothesized)
Products (Hypothesized)
®
2,4-dichlorobenzoic acid
--VAROX DCBP-50 Paste
2,4-dichlorobenzoyl peroxide
VAROX DCP dicumyl
methane; acetophenone; -methylstyrene;
peroxide
-cumyl alcohol
-methylstyrene oxide;
-cumyl methyl ether
10
VAROX® VC-Flake
Peroxide Accelerator
m/p-di(tert-butylperoxy)
diisopropyl benzene
methane; acetone; tbutyl alcohol; 1,3 & 1,4-diacetyl benzenes; 1,3 &
1,4-di(-hydroxyisopropyl)
benzenes; 1-acetyl-3 or
4-(-hydroxy-isopropyl)
benzenes
isobutene; isobutene oxide, 3 or
4-(-hydroxyisopropyl)-methyl styrene oxides; 3or
4-(-hydroxyisopropyl)-methylstyrene oxides; 3
or 4-(-hydroxyisopropyl)
-methylstyrenes; 3 or
4-(-hydroxyisopropyl)-methylstyrenes
methane; acetone; tert
isobutene; isobutene
VAROX DBPH
2,5-dimethyl-2,5-di(t-butylperoxy) butyl alcohol; tert amyl al- oxide; ethane;
hexane
cohol; ethane; 2,5-dihy2-methyl-3-butyn-2-ol;
droxy-2,5-dimethylhexane 2-butanone;
2,5-hexanedione
Isobutene; isobutene
VAROX 130-XL
methane; acetone;
2,5-dimethyl- 2,5-di(t-butyl-peroxy) tert-butyl alcohol;
oxide;
hexyne-3
2,5-dihydroxy-2,55-hydroxy-2,5-dimethyl3-hexy-1-ene oxide;
dimethyl-3-hexyne;
3-hexyne-2,5-dione; 5-hy- 2-methyl-5-oxo-3-hexyn-1droxy-2-methyl-3-hexyn-2- ene oxide; 2-methyl-3-butyn-2-ol; 3-butyn-2-one
one
VAROX 230-XL PDR
acetone; methane; tertt-butyl methyl ether
n-butyl 4,4-di(t-butylperoxy) valer- butyl alcohol; tert-butyl
ate
hydroperoxide; n-butyl
levulinate; carbon dioxide; acetic acid; n-butyl
propionate; n-butyl acrylate
VAROX 231-XL PDR
acetone; methane; t1,1,3-trimethylcyclopenbutyl alcohol; t-butyl
tane; 3,3,5-trimethylhydroperoxide; 3,3,5-trihexanoic acid; 3,5,5-trimethylcyclohexanone;
methylhexanoic acid;
carbon dioxide; 2,2,4-tri3,3,5-trimethyl-5-hexanoic
methyl-1-pentene; t-butyl acid
trimethylpentyl ethers;
2,2,4-trimethylpentylene
di-t-butyl ether
VAROX TBPB
methane; acetone; t-bu- ethane; t-butyl methyl
tyl alcohol; benzoic acid
ether
Percent Peroxide Remaining vs. Time
Figures 3 and 4 compare the different amounts of peroxide that remain at the given
temperatures of 325°F and 350°F.
11
Peroxide Remaining
100%
®
VAROX 231-XL
80%
Peroxide Accelerator
VVAROX DCPAROX
D
DCP
60%
-Flake
VVAROX VC-RAROX
40%
VVAROX DBPHROX D
20%
VVAROX 130-XLAROX
0%
0
4
8
12
16
24
20
Time (min)
28
32
36
40
Figure 3: Percent Peroxide Remaining at 325°F
Peroxide Remaining
100%
VVAROX® 231-XL
80%
Peroxide Accelerator
VVAROX DCP
DCPAROX
60%
VVAROX VC-RAROX
-Flake
40%
VVAROX DBPHAROX
20%
VVAROX 130-XLARO
0%
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Time (min)
Figure 4: Percent Peroxide Remaining at 350°F
Processing Time (Scorch)
Experimental data were generated using a Mooney viscometer, as shown in Figure 5.
The tS5 value is the scorch time at the processing temperature (usually at the polymer
extrusion temperature). This value represents the time during which the compound can
be safely processed before unwanted crosslinking or “scorch” takes place. The tS5 value
is defined as the time needed at a specific temperature to obtain a 5 Mooney unit
increase in the viscosity, as measured from the MV or minimum viscosity. This value
provides the user valuable information on process safety. It is important to note that any
premature crosslinking generated during compounding is not reversible; it can lead to an
undesirable increase in elastomer viscosity.
Scorch Time, ttsS 505(min)
(min)
25
VAROX® 231
Peroxide Accelerator
20
-Flake
VAROX VC-R
15
VAROX DCP
DCP
10
VAROX 230
5
VAROX DBPH
0
90
100
110
120
130
140
150
160
170
Temperature (°C)
Figure 5: Mooney Scorch vs. Temperature in an EPDM Compound2
12
Example: If an EPDM compound containing VAROX® VC-Flake Peroxide Accelerator is
processed at 130°C, its viscosity will be increased by 5 Mooney Units after about 10
minutes.
Cure Time (t′90)
Experimental data were recorded using an ODR 2000E rheometer. The t′90 value
represents the time needed to reach 90% of the difference between the maximum and
the minimum crosslinking density. The t′90 value is one of the key parameters used to study
improvements in productivity.
25
VAROX® 231
t ' 90 (min)
20
Peroxide Accelerator
VAROX 230
15
-Flake
VAROX VC-R
10
VAROX DBPH
5
VAROX DCP
0
130
140
150
160
170
180
190
Temperature (°C)
Figure 6: t′90 vs. Temperature in an EPDM Compound2
Example: 90% of the crosslinking density of an EPDM compound cured with
VAROX VC-Flake at 170°C will be obtained after 9 minutes. It takes 3 minutes at 185°C to
obtain the same result.
Crosslinking Efficiency
An ODR 2000E rheometer was used to generate the data in Figure 7. MH (N•m) is the
maximum torque developed in the compound, which is relative to the amount of
crosslinking bonds created by the peroxide, and is an indication of some of the
mechanical properties to be expected in the cured product.
®
M H (N•m)
(*Exception: VAROX 231Peroxide Accelerator Crosslinked @ 170ºC)
5
4.8
4.6
4.4
4.2
4
3.8
3.6
3.4
3.2
3
VAROX 231
VAROX DCP
VAROX VC-R
-Flake
VAROX DBPH
VAROX 230
1
2
3
4
5
6
Quantity of Peroxide (phr)
Figure 7: Crosslinking of EPDM at 185°C*2
Example: When curing EPDM at 185°C, data suggest that only 2.6 phr of pure
VAROX VC-Flake are required to provide the same level of crosslink density as 4.3 phr of
pure VAROX DCP.
13
Specifications
Commercial
Product
Peroxide
Class &
CAS #
Generic Name Peroxide
Structure Molecular Weight
Assay (%)
Active
Oxygen
(%)
Physical
Form
Diluent,
Filler, or
Binder
Specific Gravity
[g/cm3 at °C]
and/or Bulk
Density
VAROX®
DCP-99
Dialkyl
80-43-3
Dicumyl Peroxide
M.W. 270..37
98.0min
5.80 min
White
Crystals
---
1.04 @ 20°C
97.0 min
9.18 min
Solid
Yellow
Flakes
---
0.952 @ 25°C
CH3
C
CH3
OO C
CH3
VAROX
Dialkyl
VC-Flake
25155-25-3
(Currently not being sold)
CH3
CH3
CH3
CH3
C
C
C
C
CH3
CH3
OO
CH3
● Methane
● Acetophenone
● Cumyl Alcohol
CH3
α, α'-Di(t-butylperoxy)diisopropylbenzene
M.W. 338.48
CH3
Typical
Decomposition
Products in Inert
Media
OO
CH3
● Methane
● Acetone
● t-Butyl Alcohol
● Mixture of Aromatic
Hydrocarbons
CH3
VAROX
DBPH
Dialkyl
78-63-7
93.0 min
10.25 min
Liquid
---
0.8650 @ 25°C
VAROX
DBPH-50
Dialkyl
45.0-48.0
4.96-5.29
Free
Flowing
Powder
CaCO3
Silica
N/A
VAROX
DBPH-50-EZD
Dialkyl
45.0-48.0
4.96-5.29
Free
Flowing
Powder
Silica
N/A
45.0-48.0
4.96-5.29
Paste
Silicone
Polymer
1.09 ± 0.10
19.0-21.0
2.09-2.31
Free
Flowing
Beads
PolyPropylene
N/A
45.0-48.0
5.03-5.36
Free
Flowing
Powder
CaC3
Silica
1.26 @ 20°C
39.5-41.5
3.78-3.97
Free
Flowing
Powder
CaCO3
Micro Cel E
25.1 lbs/ft3
40.0
4.18-4.39
Free
Flowing
Powder
CaCO3
Silica
32.0 lbs/ft3
● Methane
● Trimethyl-cyclopentane, 3,3,5
● Trimethyl-cyclohexanone
● CO2
● Acetone
● t-Butyl Alcohol
49.0-51.0
2.06-2.19
Paste
Silicone Oil
1.20 @ 20°C
● 2,4 dichlorobenzoic
acid
● Carbon Dioxide
● Carbon Monoxide
98.0
8.08 min
Liquid
---
1.04 @ 25°C
49.5-51.5
4.08-4.24
Free
Flowing
Powder
CaCO3
Silica
N/A
2,5-dimethyl-2,5-Di(t-butylperoxy)hexane
M.W. 290.44
CH 3
VAROX
DBPH-50 SG
VAROX
DBPH-P20
VAROX
130-XL
Dialkyl
CH 3
CH 3
CH 3
CH 3 C OO C CH 2 CH 2 C OO C CH 3
CH 3
CH 3
CH 3
CH 3
Dialkyl
2,5-dimethyl-2,5-Di(t-butylperoxy)hexyne-3
M.W. 286.41
Dialkyl
CH3
CH3
CH3
C OO C C C C OO C CH3
CH3
VAROX
Peroxyketal
230-XL
995-33-5
(Currently not being sold)
CH3
CH3
CH3
CH3
O
OO C(CH3)3
O C (CH2)2 C
OO C(CH3)3
VAROX
231-XL
Peroxyketal
6731-36-8
1,1 bis-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane
M.W. 302
CH3
CH3
C
CH3
OO
OO
CH3
VAROX
DCBP-50
Paste
CH3
C
Peroxyester
O
O
Cl
C
● Methane
● Butyl Propionate
● Butyl Levulinate
● Carbon Dioxide
● Acetone
● t-Butyl Alcohol
Cl
Cl
VAROX
TBPB-50
O
O
Cl
Peroxyester
614-45-9
CH3
Di-(2,4 dichlorobenzoyl)
peroxide
Diacyl
133-14-2
VAROX
TBPB
C
CH3
CH3
CH3
● Methane
● Acetone
● t-Butyl Alcohol
CH3
n-Butyl 4,4-Di(t-butylperoxy)valerate
M.W. 334
nC4H9
● Ethane
● Methane
● Acetone
● t-Butyl Alcohol
● Mixture of Aromatic
Hydrocarbons
t-butyl Perbenzoate
M.W. 194
O
CH 3
C O O C CH 3
CH 3
14
● Methane
● Acetone
● t-butyl Alcohol
● Benzoic Acid
SBR
0.7 - 1.5
0.4 -1.0
0.4 1.5
1.6 2.7
0.3 - 0.6
---
0.6 - 1.2
---
---
1.8 - 4.0
2.0 - 4.5
2.0 - 4.5
---
1.6 2.7
1.6 2.7
Q
0.4 - 0.8
0.2 -1.0
0.4 0.8
0.5 1.0
-----
1.1 -2.3
---
---
0.9 - 2.0
---
0.5 1.0
0.5 1.0
NR or IR
0.8 - 1.6
0.5 -1.5
0.8 1.6
2.0 4.0
-----
---
2.2 - 4.3
2.5 - 5.0
2.5 - 5.0
---
2.0 4.0
2.0 4.0
NBR
0.9 - 1.7
0.5 -1.5
1.1 2.0
2.4
4.4
-----
---
2.5 - 5.0
3.0 - 5.5
3.0 - 5.5
---
2.4
4.4
2.4
4.4
PP
-----
-----
---
---
---
3.0 - 5.5
1.0 - 2.0
---
---
0.1 1.0
---
LDPE
1.5 - 2.5
1.2 -1.8
1.4 2.0
------4.0 - 8.0
-------
--7.5 - 13
7.5 - 13.0
3.5 - 6.5
-----
---
HDPE
HNBR
2.2 - 4.2
--2.5 4.7
5.5 10.5
5.5 10.5
5.5 10.5
----0.5 1.2
------1.5 - 3.0
-----
---
-------
257°F (125°C)
*The suggested maximum
compounding temperature
is the temperature at which
the scorch time is equal to
5 minutes1
---
265-345°F
(129-173.8°C)
---
257°F
(125°C)
---
291.2°F
(144°C)
---
339.5°F
(170.8°C)
FKM
257°F
(125°C)
---
265-345°F
(129-173.8°C)
0.8 -1.6
257°F
(125°C)
0.5 1.2
291.2°F
(144°C)
1.2 2.5
339.5°F
(170.8°C)
1.2 2.5
167°F
(75°C)
1.2 2.5
230-266°F
(110-130°C)
---
162°F
(72°C)
---
192°F
(89°C)
---
230°F
(110°C)
---
221°F
(105°C)
---
280-360°F
(138-182°C)
---
239°F
(115°C)
---
268°F
(131°C)
EVA
307°F
(153°C)
1.2 - 2.0
230°F
(110°C)
0.8 -1.6
266-347°F
(130-175°C)
1.0 2.0
263.8°F
(128.8°C)
2.0 5.0
294.3°F
(145.7°C)
2.0 5.0
337.6°F
(169.8°C)
2.0 5.0
305.6°F
(152°C)
---
340-420°F
(171-215.5°C)
3.5 - 6.5
306°F
(152°C)
3.5 - 6.5
336°F
(169°C)
4.0 - 5.8
381°F
(194°C)
---
293°F
(145°C)
---
320-400°F
(160-204°C)
---
284°F
(140°C)
EPM/
EPDM
315°F
(157°C)
2.4 - 5.4
358°F
(181°C)
1.6 - 3.4
293°F
(145°C)
1.7 3.4
320-400°F
(160-204°C)
3.8 7.6
284°F
(140°C)
3.8 7.6
315°F
(157°C)
3.8 7.6
358°F
(181°C)
---
293°F
(145°C)
7.5 - 13.0
320-400°F
(160-204°C)
7.5 - 13.0
284°F
(140°C)
3.5 - 6.5
315°F
(157°C)
---
358°F
(181°C)
---
293°F
(145°C)
---
320-400°F
(160-204°C)
0.8 - 1.6
284°F
(140°C)
0.5 -1.0
315°F
(157°C)
0.5 1.4
358°F
(181°C)
1.0 3.0
293°F
(145°C)
1.0 3.0
320-400°F
(160-204°C)
1.0 3.0
284°F
(140°C)
---
315°F
(157°C)
1.0 - 3.0
358°F
(181°C)
1.1 - 3.5
282°F
(139°C)
0.5 - 2.0
320-400°F
(160-204°C)
---
282°F
(139°C)
---
315°F
(157°C)
---
358°F
(181°C)
2.4 - 3.8
266°F
(130°C)
1.5 - 2.4
310-390°F
(154-199°C)
2.5 4.0
279°F
(137°C)
5.6 8.9
315°F
(157°C)
5.6 8.9
352°F
(178°C)
5.6 8.9
Suggested
Maximum
Compounding
Temp*1 F(°C)
---
Suggested
Cure Temps
F(°C)
5.6 - 8.9
1 Hr
Half-Life
Temp
F(°C)
3.5 - 6.5
10 Min
Half-Life
Temp
F(°C)
7.5 - 14.0
1 Min
Half-Life
Temp
F(°C)
CR
Compounding Information
CPE
Half Life Temperatures
15
Effect of Compounding Ingredients
Antidegradants ─ One class of free radical scavengers consists of antidegradants. The
amount of cure inhibition they exhibit depends on the particular chemical. Figure 8 shows
the effects of various antioxidants in a formulation containing 100 phr of EPDM, dicumyl
peroxide as indicated, and 0.5 phr of antioxidant.
100
No Antioxidant
MH-ML(dN•m)
80
60
Quinoline
40
Amine Type
20
Hindered Phenol
0
1
2
3
Dicumyl Peroxide (phr)
Figure 8: Effects of Various Antioxidants 5
As Figure 8 demonstrates, when using an amine antioxidant, it is necessary to use three
phr of dicumyl peroxide to obtain the same state of cure as with one phr of dicumyl with
no antioxidant. The quinoline antioxidant has the least effect on the state of cure,
followed by the amine, while the hindered phenol antioxidant severely reduces the final
cure. Antiozonants of the p-phenylenediamine type will reduce peroxide efficiency to
the greatest extent. The best antioxidants for use with peroxide cures are AGERITE® RESIN
D® Antioxidant, METHYL NICLATE® Antioxidant, and VANOX® MTI Antioxidant and VANOX
ZMTI. Typically, sensitivity to this effect decreases in the following order: VAROX® 231
Peroxide Accelerator > VAROX DCP > VAROX VC-Flake > VAROX DBPH > VAROX 130.
MH-ML (dN•m)
Plasticizers ─ Are additives used as processing aids, extenders (to lower the compound
cost), and as active ingredients capable of imparting special properties to vulcanizates.
Some plasticizers, especially aromatic oils, are not recommended because they can
consume a portion of the radicals generated by the peroxides. In this regard, paraffinic
type oils are preferred. Figure 9 shows the effect of various plasticizing oils in an EPM
formula consisting of 25 phr of oil, and 2.2 phr of VAROX 130-XL.
30
20
10
0
No Oil
n-Decane
Paraffinic
DOP
Naphthenic
Figure 9: Effects of Various Plasticizing Oils5
16
Typically, sensitivity to this effect decreases in the following order: VAROX® DBPH Peroxide
Accelerator > VAROX VC-Flake > VAROX DCP > VAROX 231 > VAROX 130.
Fillers ─ Reinforcing and nonreinforcing fillers such as carbon black, silicates, silica, kaolin
clay and calcium carbonate can be used in compounds cured with peroxides. However,
acidic fillers such as “channel” carbon blacks, “hard clay” and “acidic silicas” can initiate
ionic decomposition of the peroxide. Different peroxides are sensitive to acidic materials to
varying degrees. The peroxyketals such as VAROX 231 are perhaps the most sensitive;
VAROX DCP is somewhat less sensitive, followed by VAROX DBPH. VAROX 130 is the least
sensitive to acidic fillers. If the use of acidic fillers is necessary, it is advisable to neutralize
the compound with small quantities of basic metallic oxides (MgO, ZnO), or with amines
(DPG, hexamethylene, tetramine, triethanolamine).
Coagents
Coagents containing unsaturation can help increase the crosslink density. Coagents
become part of the crosslink network, and can also affect the cure characteristics. Table 9
highlights common coagents available today.
Table 9: Common Coagents6
Description
Difunctional Liquid Methacrylate
Trifunctional Liquid Methacrylate
Scorch-Retarded Liquid Dimethacrylate
Scorch-Retarded Liquid Trimethacrylate
Scorch-Retarded Liquid Triacrylate
Scorch-Retarded Liquid Dimethacrylate
Scorch-Retarded Solid Diacrylate
Scorch-Retarded Metallic Diacrylate
Trade Name
SR 297 (BGDMA)
SR 350 (TMPTMA)
Saret® SR 516
Saret SR 517
Saret SR 519
Saret SR 521
Saret SR 522
Saret SR 633
Saret 75 EPM 2A (75% active)
Saret SR 634
Saret 75 EPM 2M (75% active)
VANLINK™ TAC Coagent
VANAX®MBM Accelerator
Ricon® 100
Ricon 153
Ricon 153 D (65% active)
Ricon 154
Ricon 154 D (65% active)
Ricobond® 1731
Ricobond 1731 HS (69% active)
Ricobond 1756
Ricobond 1756 HS (69.5% active)
Scorch-Retarded Metallic Dimethacrylate
Triallyl Cyanurate
Bis-maleimide
Styrene/Butadiene Copolymer (70% vinyl)
85% Vinyl Liquid Polybutadiene
90% Vinyl Liquid Polybutadiene
Maleinized Liquid Polybutadiene (28% vinyl)
Maleinized Liquid Polybutadiene (70% vinyl)
Acrylates, methacrylates, and maleimides are classified as Type 1 coagents which
typically shorten scorch time in addition to increasing the state of cure. Type 2 coagents
such as polybutadiene, VANLINK™ TAC Coagent, and VANLINK™ TAIC Coagent increase
17
efficiency without significantly increasing the cure rate. While Type 1 coagents have the
advantage of a faster cure rate, they also have a higher tendency to scorch. Figure 10
compares scorch values for common Type 1 acrylate/methacrylate coagents (Saret® SR)
with those of several Type 2 liquid polybutadiene coagents (Ricon®/Ricobond®) – at a
level of 5 parts per 100 of rubber in peroxide-cured EPDM.
®
7.5 phr VAROX DCP-40KE Peroxide Accelerator + 5 phr Coagent
4
3.5
tS2 (min)
3
2.5
2
1.5
1
Ricon 100
Ricon 154
Ricon® 153
Ricobond® 1756
Saret SR 517
Saret SR 521
Saret SR 522
Saret 75
EPM 2A
Saret SR 75
EPM 2A
0
Saret ®SR 519
0.5
Figure 10: Scorch Comparison of Coagents in EPDM6
The key to an ideal cure is the correct choice of a proper coagent/peroxide system.
The following pages describe the benefits coagents can provide in peroxide-cured
compounds.
Improved Efficiency of Cure ─ Although all coagents will increase the efficiency of cure
to some degree, there are several coagents which give the greatest boost in crosslink
density. These include SR 350, SR 517, SR 519, Ricon 154, VANLINK™ TAC Coagent, and
VANAX® MBM Accelerator. Figure 11 illustrates the effectiveness of a bis-maleimide
coagent.
Cure: 7.5 phr VAROX® DCP-40KE Peroxide Accelerator
Accelartor +
®
VANAX MBM Accelerator
100% Modulus (psi)
600
500
400
300
200
100
0
0
1
2
Coagent Level (phr)
Figure 11: Modulus Response of Bis-Maleimide in EPDM6
18
3
Higher Tear Strength ─ The tear strength of peroxide-cured systems is usually
considered inferior to that of sulfur cures. Figure 12 illustrates hot tear values at 150°C in
EPDM for several coagents as well as a sulfur/accelerator control.
160
Sulfur Control: 2 phr Sulfur, 1 phr CAPTAX ®Accelerator,
1.5 phr VANAX® TMTM Accelerateor
DC
Proxide Cure: 4.5 phr of VAROX®CDP-40KE
Peroxide Accelerator + C
coagent
Tear Strength (psi)
140
120
100
80
60
40
Saret 75
EPM 2A
Ricon 154
Saret SR 521
Ricobond® 1756
Ricon 100
Ricon® 153
Saret SR 519
Sulfur*
Saret 75
EPM 2A
Saret SR 522
0
Saret ®SR 517
20
Figure 12: Coagent Tear Strength Response in EPDM6
Improved Heat Aging ─ It is well known that peroxide-cured systems have superior heat
aging as compared to sulfur systems. The addition of a coagent to the peroxide-cured
formulation maintains excellent heat aging properties compared to sulfur, as shown in
Figure 13.
80%
Sulfur Control: 3 phr Sulfur, 1 phr CAPTAX ® Accelerator,
®
3 phr VANAX TMTM Accelerator
Accelerator
Peroxide Cure: 7.5 phr VAROX® DCP-40KE Peroxide Accelerator,
15 phr Saret®633, 20 phr Saret 634
Percent Change
60%
40%
20%
0%
-20%
-40%
-60%
-80%
ZDMA (Saret 634)
ZDA (Saret 633)
Elongation
Modulus
Sulfur
-13%
-20%
-62%
16%
7%
63%
Figure 13: Heat Aged Elongation and Modulus in EPDM6
19
Improved Compression Set ─ Liquid acrylate and polybutadiene coagents can be used
to obtain improved compression set values. Figure 14 compares the compression set of
several acrylate, methacrylate, and polybutadiene coagents to that of a peroxide
control.
Peroxide Control: 7.5 phr VAROX
VARPX®DCP-40KE Peroxide Accelerator
60
Percent Set
50
40
30
20
Peroxide*
Ricon 100
Ricon 154
Ricon 150
Saret SR 517
Saret SR 516
Ricon® 153
Saret ®SR 522
Saret SR 519
0
Saret SR 521
10
Figure 14: EPDM Compression Set with 5 phr Coagent6
Lower Mooney Viscosity ─ Liquid coagents can be termed “reactive plasticizers”.
They lower the viscosity of a formulation during processing, and add significant
crosslinking upon vulcanization. By using these coagents, process oils and other
extractable plasticizers can be reduced or even eliminated in some cases. Figure 15
illustrates the plasticizing effect in a non-oil-filled polyisoprene system.
Cure: 4 phr VAROX® DCP-40KE Peroxide Accelerator + Coagent
80
(4 min. @ 100°C)
Mooney Viscosity
70
60
50
Saret® SR 517
40
Ricon®153
30
20
10
0
0
2
5
10
Coagent Level (phr)
Figure 15: Coagent Viscosity Response in Polyisoprene6
20
Improved Rubber to Metal and Rubber to Fiber Adhesion ─ Several coagents will
increase a peroxide-cured rubber compound’s adhesion to various metallic and fibrous
substrates. These include Saret® 633, Saret 634, Saret 75 EPM 2A, Saret 75 EPM 2M,
Ricobond® 1756, Ricobond 1756 HS, Ricobond 1731, and Ricobond 1731 HS. These
coagents can be used alone or as blends (Saret/Ricobond) to achieve excellent
adhesive properties. Table 10 shows the advantages of a blend of Saret 633 and
Ricobond 1756 when a flexible, yet tough, bond to steel is required.
Table 10: Coagents and Adhesion to Steel
Compound (phr)
1
2
256.0
256.0
Vistalon™ 2504 & Vistalon™ 7500 (50:50) MB
1.0
1.0
AGERITE® RESIN D® Antioxidant
®
7.5
7.5
VAROX DCP-40KE Peroxide Accelerator
®
5.0
Saret 633
5.0
Ricobond® 1756
Totals
269.5
269.5
Physical Properties
T-Peel Adhesion (cold roll steel), lbs
Lap Shear Adhesion (cold roll steel), psi
44
1720
76
1279
3
256.0
1.0
7.5
2.5
2.5
269.5
69
1792
Improved Dynamic Properties ─ Metallic diacrylate and dimethacrylate coagents can
be used to improve dynamic properties such as tan delta and dynamic flex. Figure 16
shows the advantages of using SR 633 and SR 634 in a dynamic flex application in
synthetic polyisoprene.
®
Sulfur Control: 1.6phr Sulfur, 106 phr VANAX NS Accelerator
Peroxide Cures: 2 phr VAROX® DCP-40KE Peroxide Accelerators +5 phr Coagent
200
150
100
Saret SR 634
Saret SR 633
Saret ®SR 517
Ricon® 153
Sulfur
0
Peroxide
50
Ricobond® 1756
Cycles to Failure x 1000
250
Figure 16: DeMattia Flex Response of Polyisoprene with Coagent6
Several other performance advantages can be obtained by the use of a coagent.
These include: improved resistance to oils and fuels, higher tensile strength, increased
hardness and enhanced abrasion resistance.
21
SILICONE RUBBER AND PEROXIDES
Two specific peroxides are preferred for crosslinking silicone rubber: “non-vinyl specific”
VAROX® DCBP-50 Paste Peroxide Accelerator, and “vinyl specific” VAROX DBPH.
Dimethyl Polysiloxane (MQ)
This type of silicone rubber, also known as polydimethylsiloxane, has the general
structure shown below, where n = 3,000 to 10,000 units.
CH3
Si
CH3
CH3
O
CH3
Si
O
Si
CH3
n
C H3
MQ Silicone
Polydimethylsiloxane does not contain any double bonds or unsaturation and must be
cured by hydrogen abstraction of the labile hydrogens on the pendant methyl groups. A
great deal of energy is required to remove a hydrogen from the methyl group (CH3) of an
MQ elastomer, and very few peroxides are capable of effectively crosslinking this
polymer. VAROX DCBP-50 Paste (dichlorobenzoyl peroxide), although having lower
thermal stability than the dialkyl and peroxyketal peroxides, produces very high energy
radicals (112 kcal/mole) that can abstract a hydrogen from the pendant methyl group
to effectively crosslink MQ. Dichlorobenzoyll peroxide, which is a member of the diacyl
peroxide class, is often referred to as a “non-vinyl specific” peroxide for this reason.
Methyl Phenyl Polysiloxane (PMQ)
Manufacturers of PMQ advise that the additional phenyl groups (benzene rings) in PMQ
improve low temperature flexibility and resistance to gamma radiation. This polymer also
requires VAROX DCBP-50 Paste for crosslinking.
C H3
Si
C H3
C H3
O
Si
C H3
O
Si
C H3
Methyl Phenyl Polysiloxane (PMQ)
22
The Vinyl Containing Silicone Elastomers:
• Methyl Vinyl Polysiloxane (VMQ)
• Fluoro Methyl Vinyl Polysiloxane (FVMQ)
• Methyl Phenyl Vinyl Polysiloxane (PVMQ)
C H3
Si
C H3
C H3
O
Si
C H3
C H3
O
Si
Si
C H3
C
H
O
Si
H
C H2
C H2
H
C F3
C F3
C
Methyl Vinyl Polysiloxane (VMQ)
C H3
Si
C H3
O
Si
C
H
H
C
H
Fluoro Methyl Vinyl Polysiloxane (FVMQ)
C H3
O
C H3
C H3
Si
O
C H3
C H3
Si O
Si
C H3
C H3
O
C H3
Si
H
C
C
H
H
Methyl Phenyl Vinyl Polysiloxane (PVMQ)
The pendant vinyl groups in these rubbers are quite reactive to both low and high
energy free radicals. The presence of the vinyl groups in the VMQ, FVMQ and PVMQ
greatly increases the peroxide crosslinking efficiency, so that all the peroxides used for
crosslinking can cure these elastomers. However, not all peroxides are suitable for
crosslinking vinyl containing silicone and fluorosilicone elastomers.
Important considerations in the selection of peroxide are: no bloom, no color
formation, non-aromatic decomposition products, low odor, and FDA indirect food
contact clearance. VAROX® DBPH Peroxide Accelerator [liquid 2,5-dimethyl-2,5-di-(tbutyl-peroxy)hexane and its extended forms] continues to be the favorite choice with
regard to these properties.
C H3
C H3
C H3
C H3
C H3
C
C
C
C
OO
C H3
C H 2 C H2
C H3
OO
C H3
C H3
C H3
2,5-dimethyl-2,5-di(t-butylperoxy)hexane (VAROX DBPH)
Peroxides for crosslinking silicone can be split into two classes: “vinyl specific” and
“general purpose”. These peroxides and their application to silicone are described in
Table 11 on the following page.
23
Table 11: Peroxide Applications in Silicone Rubber7
VAROX® DCP
Peroxide Accelerator
(Dicumyl
peroxide and
extended grades)
VAROX VC-Flake
(1,3/1,4-di
(tertbutylperoxy-isopropyl)– benzene)
VAROX 130-XL
(2,5-dimethyl-2,5-di-tertbutylperoxy-3-hexyne)
Typical
Curing
Temp (°C)
Dosage
(phr)
Commercial &
(Chemical) Names
Type
Compounding
Information
VS
1.1
to
2.2
150
to
>200
VS
0.3
to
0.9
150
VS
0.4
to
1.5
>150
Application Technology
Requires a higher curing temperature than
general purpose peroxides, and is unsuitable for hot air or UHF (microwave) curing.
Normally used for molding, autoclave and
continuous (steam or salt bath) vulcanization of insulation and tubing products.
Does not form acidic decomposition products, so cure products do not require a
postcure. Since it melts at approximately
40°C, good dispersion can be obtained by
mixing at temperatures above the M.P.
Acetophenone, a decomposition product,
imparts a strong odor to the cured product,
which can be reduced by postcure.
Very efficient crosslinker that is used instead
of dicumyl because of its lesser odor.
High thermal stability peroxide used primarily for curing elastomers that must be mixed
at elevated temperatures.
24
Table 11: Peroxide Applications in Silicone Rubber7 continued
VAROX® DBPH
Peroxide Accelerator
VS
0.4
to
1.5
GP
0.3
to
0.6
(2,5-dimethyl-2,5-di-tertbutylperoxy-hexane)
VAROX TBPB
(Tert-butylperoxy
benzoate)
VAROX DCBP-50 Paste
(Di(2,4-dichlorobenzoyl) GP
peroxide)
1.1
to
2.3
Typical
Curing
Temp (°C)
Dosage
(phr)
Commercial &
(Chemical) Names
Type
Compounding
Information
160
to
205
140
90
Application Technology
Characterized by high thermal stability.
Complies with FDA 21 CFR 177.2600. Liquid
at room temperature does not present any
dispersion problems in silicone compounds.
Because it is somewhat volatile, its silicone
compounds should not be stored for long
periods at relatively high temperatures.
Has excellent scorch stability and is
recommended for applications where UV
stability and transparency are required.
Excellent processing safety. Used where
scorch resistance is required.
Used for low temperature curing of silicone
compounds. Can be cured without
external pressure because of its low
activation temperature (can be scorchy).
Suitable for continuous hot air curing. Not
suitable in carbon black-filled compounds.
Postcure is required to prevent acidic decomposition of the rubber product.
25
FDA Compliance: Peroxide in Indirect Food Additives
On the next page substances are listed in FDA regulations covering polymers, resins,
paper products, coatings or adhesives intended for food packaging or food-contact
applications in accordance with Title 21, U.S. Code of Federal Regulations (21 CFR), as
amended.
Chemical &
Regulation
(Commercial) Names
Limitations1
Benzoyl peroxide2
Peroxide Accelerator
None.3
For use only as a preservative in paper coating compositions and limited to use at a level
not to exceed 0.01 mg/in2 (0.0016 mg/cm2) of the finished paper and paperboard.
For use as a catalyst in the production of crosslinked polyester resins for repeated contact
with food; total catalysts not to exceed 1.5 percent.4
For use as a vulcanization accelerator in rubber products for repeated contact with food;
total vulcanizing accelerators not to exceed 1.5 percent by weight of rubber product.
Generally recognized safe for use in food-contact applications subject to any limitations in
parts 174, 175, 176, 177, 178, 186 or §179.45 of Chapter 1.
§175.105(c)(5)
§176.170(a)(5);
§176.180(b)(1)
§177.2420(b)3
§177.2600(c)(4)(ii)(b)
§184.1(a)
Di-tert-butyl peroxide
§177.2600(c)(4)(ii)(b)
For use as a vulcanization accelerator in rubber products for repeated contact with food;
total vulcanizing accelerators not to exceed 1.5 percent by weight of rubber product.
tert-Butyl hydroperoxide
§175.105(c)(5)
§176.170(a)(5);
§176.180(b)(1)
None.3
For use only as a polymerization catalyst in the production of paper and paperboard.
tert-Butyl peracetate
§177.2600(c)(4)(ix)
Total substances listed in paragraph (c)(4)(ix) not to exceed 5 percent by weight of rubber product when used as an adjuvant substance in the production of rubber articles for
repeated contact with food.
p-tert-Butyl perbenzoate
[VAROX TBPB]
§175.300(b)(3)(xxxii);
§175.390(b)(2)
For use as a catalyst for epoxy resin in side seam cements.
Cumene hydroperoxide
§175.105(c)(5)
§176.170(a)(5)
§176.180(b)(1)
§177.2420(b)(3)
None.3
For use only as a polymerization catalyst in the production of paper and paperboard.
For use as a catalyst in the production of crosslinked polyester resins for repeated contact
with food; total catalysts not to exceed 1.5 percent.4
Dicumyl peroxide
[VAROX DCP]
§175.105(c)(5)
§175.300(b)(3)(xxxii);
§175.390(b)(2)
§177.2420(b)3
§177.2600(c)(4)(ii)(b)
None.3
For use as polymerization catalyst in side seam cements.
For use as a catalyst in the production of crosslinked polyester resins for repeated contact
with food; total catalysts not to exceed 1.5 percent.4
For use as a vulcanization accelerator in rubber articles for repeated contact with food;
total vulcanizing accelerators not to exceed 1.5 percent by weight of rubber product.
Lauroyl peroxide
§175.105(c)(5)
§177.2420(b)3
None.3
For use as a catalyst in the production of cross-linked polyester resins for repeated contact
with food; total catalysts not to exceed 1.5 percent.4
2,5-Dimethyl-2,5-di
(tert-butyl peroxy) hexane
[VAROX DBPH];
complying
1,1,4,4-Tetramethyltetramethylene)bis
[tert-butyl peroxide]
§177.1520(b)
For use as an initiator in the production of propylene homopolymer complying with
§177.1520(c), Item 1.1 and olefin copolymers complying with §177.1520(c), Items 3.1 and 3.2
and containing not less than 75 weight percent of polymer units derived from propylene,
provided that the maximum concentration of tert-butyl alcohol in the polymer does not
exceed 100 parts per million, as determined by an FDA method titled “Determination of tertButyl Alcohol in Polypropylene.”
For use as a vulcanization accelerator in rubber articles for repeated contact with food;
total vulcanization accelerators not to exceed 1.5 percent by weight of rubber product.
Methyl ethyl ketone
§177.2420(b)3
§177.2600(c)(4)(ii)(b)
For use at up to 2 percent as the sole catalyst in the production
peroxide of crosslinked polyester resins for repeated contact with food.
1 The limitations listed in this summary are those applied by the regula-
tion to the specific organic peroxide. Some regulations impose additional
limitations on finished products, such as the maximum quantity of material
that may be extracted. Please consult the individual regulations for further
information.
2 Benzoyl peroxide that meets the appropriate Food Chemicals Codex
specifications has also been affirmed as generally recognized as safe
(GRAS) for use as a bleaching agent in certain foods (i.e., flour, whey and
milk used in the production of certain cheeses). See 21 C.F.R. §184.1157
3 Section 175.105 requires food packaging adhesives produced from
the substances listed in the regulation to be separated from food by a
functional barrier. Alternatively, the quantity of adhesive contacting packaged aqueous and fatty foods must not exceed the trace amounts at
seams and at the edge exposure between packaging laminates that may
occur within the limits of good manufacturing practice; the quantity of
adhesive that contacts packaged dry food must not exceed the limits of
good manufacturing practice.
4 Limits of addition expressed as percent by weight of finished resin.
26
PEROXIDE SAFETY CHECKLIST8
The following checklist is provided as a summary of measures that will promote the safe
storage, handling and use of peroxides. The list is a basic safety and information guide,
pertaining to all organic peroxides.
1.
Different classes of organic peroxides have their own particular characteristics,
specifications and handling requirements. These are identified on the product
labels and described in the appropriate bulletins and MSDSs. The product label is
designed to indicate the recommended storage temperature, specific product
hazard characteristics, special handling information and appropriate first aid
instructions. Product bulletins provide chemical composition data, sales
specifications (including shelf life), physical properties, and safety information such
as storage temperature and SADT (Self Accelerating Decomposition Temperature).
2.
One of the most important factors to observe when working with an organic
peroxide is the recommended storage temperature. Exposure to a temperature
that can cause accelerated decomposition may result in the generation of
flammable gasses, and in some cases spontaneous ignition.
3.
Refer to NFPA 432 for storage guidelines. Proper storage is critical to the safe
handling of organic peroxides, both those normally stored at ambient temperatures
and those requiring controlled temperature storage. Ventilation is important
because air circulation around peroxides stored at low temperatures reduces the
chance of localized hot spots that can cause decomposition.
4.
Storage areas for peroxides should have explosion proof electrical equipment.
5.
Organic peroxide inventories should be rotated to avoid shelf life problems.
6.
Any observable gassing or distortion of the container should be handled very
carefully. Visible gassing of organic peroxide containers may be an indication of
imminent, possibly violent, decomposition.
7.
Only minimal quantities of peroxides should be kept in the immediate processing
area.
8.
Avoid contact with incompatible materials, such as oxidizers, reducing agents,
promoters, acids or bases.
9.
The safe use of organic peroxides requires that good housekeeping procedures be
meticulously practiced.
10.
Heat, flame, contamination, shock, friction, and static electricity are potential
hazards when an organic peroxide is being charged to a reaction. Care should be
taken to eliminate or minimize all of these.
27
11.
Polymeric materials that may be soluble in organic peroxide solutions, as well as
brass, copper and iron, should not be used in reaction or storage vessels, including
piping and valving. Compatible construction materials include stainless steel 304 or
316 (preferred), HDPE, polytetrafluoroethylene, and glass linings.
12.
Contaminants, such as iron or dirt, should be avoided when charging peroxides.
13.
Pumps used for organic peroxides must be “dedicated” to avoid potential
contamination.
14.
Static buildup can be minimized by proper grounding and by keeping free fall
distances to a minimum, especially when working with initiators sensitive to static,
such as dry benzoyl peroxide or di-t-butyl peroxide.
15.
Friction caused by pumping increases the temperature of the pumping solutions.
Extra care should be exercised when peroxide solutions are being re-circulated to
avoid temperatures above the SADT (Self Accelerating Decomposition
Temperature).
16.
When peroxide samples are used in analytical work, care should be exercised to
avoid any contamination. Clean, dry plastic or glass containers should be used to
transfer peroxide samples. Dry ice should be available to cool samples in an
emergency. Direct heat should never be applied to organic peroxides.
17.
As a rule, dilution of pure peroxides with compatible solvents will assist the safe
handling of peroxides.
18.
Any spilled organic peroxides should be attended to immediately. Spills can
normally be handled by spreading an inert absorbent substance directly on the
spill, wetting with water, sweeping the area and placing the sweepings in
polyethylene bags for appropriate disposal.
19.
Where spills occur, allow for sufficient ventilation to aid in the removal of fumes that
may be present.
20.
In disposing of organic peroxides, or the absorbent material that has been used to
remove spills, extreme care should be exercised. The wetted absorbent material
should be placed in a plastic bag and incinerated. Federal, state and local laws,
and environmental regulations, must be observed in choosing a disposal method.
21.
The procedure for disposal of empty peroxide containers must include rinsing with
water or a compatible solvent. This is especially important for refrigerated
products. In accordance with federal, state and local regulations, these can then
be sent to a landfill or incineration site.
22.
Drums must always be thoroughly flushed and drained before being sent to a
reconditioner.
23.
Cutting torches should never be used on empty peroxide drums. Flammable
vapors may be present.
28
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
The Vanderbilt Rubber Handbook, 14th Edition, 2010, Ch. 3.
Chemical Curing of Elastomers and Crosslinking of Thermoplastics, Arkema Inc.,
April, 1993.
Luperox – Crosslinking Rubber and Polyethylene, Arkema Inc., 2003.
Organic Peroxides – Product bulletin on Diacyl Peroxides, Arkema 2009
Encyclopedia of Polymer Science and Engineering, Volume 11, Second Edition,
1998.
Palys, L., Callais, P., Novits, M., and Moskal, M., Selection and Use of Organic
Peroxides for Crosslinking, Arkema Inc., ACS Rubber Meeting, May 5-8, 1998.
McElwee, C., Coagent Selection for Rubber Applications, Sartomer, ACS Rubber
Meeting, 2002.
Nijhof, L.B.G.M., Cubera, M., Peroxide Crosslinking of Silicone Compounds, ACS
Rubber Meeting, October 17-20, 2000.
Organic Peroxides – Their Safe Handling and Use, Arkema Inc. 2009.
Trademarks
AGERITE Antioxidants, CAPTAX Accelerators, NICLATE Antioxidants, UNADS Accelerators,
VANAX Accelerators, VANOX Antioxidants, and VAROX Peroxide Accelerators are
registered trademarks of R.T. Vanderbilt Holding Company, Inc. and/or its respective
wholly owned subsidiaries.
VANLINK™ Coagent is a trademark of R.T. Vanderbilt Holding Company, Inc. or its
respective wholly owned subsidiaries.
Exact is a registered trademark of Exxon Mobil Corporation.
Resin D is a registered trademark of Emerald Polymer Additives, LLC.
Responsible Care is a registered trademark of the American Chemistry Council.
Ricobond is a registered trademark of Sartomer Technology Company, Inc.
Ricon is a registered trademark of Sartomer Technology Company, Inc.
Saret is a registered trademark of Sartomer Technology Company, Inc.
Sartomer is a registered trademark of Sartomer Technology Company, Inc.
Vamac® is a registered trademark of E. I. Du Pont De Nemours and Company.
Vistalon is a trademark of ExxonMobil Corporation.
Vistamaxx is a registered trademark of Exxon Mobil Corporation.
Viton® is a registered trademark of DuPont Performance Elastomers LLC.
29
Vanderbilt Chemicals, LLC
30 Winfield Street, P.O. Box 5150
Norwalk, CT 06856-5150
(203) 853-1400 Fax: (203) 838-6368
E-Mail: rubber@vanderbiltchemicals.com
vanderbiltchemicals.com
Vanderbilt Chemicals, LLC
6281 Beach Boulevard, Suite 204
Buena Park, California 90621
(714) 670-8084 Fax: (714) 739-1488
E-Mail: westcoastoffice@vanderbiltchemicals.com
vanderbiltchemicals.com
hemic
ilt C
a
rb
,
ls
LLC
Van
de
Please visit our website for sample requests, sales specifications, and
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UL
IS
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0 1:2
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24
6
rev09/17/2014